1. Field of the Invention
The present invention relates generally to image signal processing and specifically a luminance/chrominance signal separating technique utilizing motion estimation and compensation. In particular, the present invention relates to a three-dimensional (3-D) luminance/chrominance (Y/C) signal separating device which can provide a high quality image by selectively performing one of a 3-D Y/C signal separation, motion-compensated 3-D Y/C signal separation, and Y/C signal separation per block using a 2-D field comb filter in accordance with the detected image motion.
2. Description of the Prior Art
Conventionally, in the composite video signals of the National Television System Committee (NTSC) system which has been adopted in many countries in the world, the chrominance (C) signal is modulated into a chrominance subcarrier and then interleaved in spectral spaces existing between the frequency spectrums of the luminance signal so as to be compatible with black/white television signals.
In order to separate the luminance signal and the chrominance signal interleaved by the frequency multiplication in the NTSC system, a horizontal filter was first introduced, and subsequently a one-line vertical filter and a two-line vertical filter have been used as the Y/C signal separating device. Recently, a motion-compensated 3-D comb filter using frame memories has been commercialized as a large scale integrated circuit (LSI) and gradually applied to a high definition television.
The horizontal filter may be classified into a horizontal low-pass filter and a horizontal band-pass filter. In the Y/C signal separating device using the horizontal low-pass filter, the Y signal is first separated from the composite video signal through the low-pass filter to remove the low frequency band of the chrominance subcarrier, and then the separated luminance component is subtracted from the composite video signal to separate the chrominance signal.
In the Y/C signal separating device using the line vertical filter, the Y/C signal separation is performed utilizing the characteristic of the phase inversion for each line of the composite video signal.
In the Y/C signal separating device using the motion-compensated 3-D comb filter, the Y/C signal separation is performed by properly mixing the output of a 2-D Y/C signal separating filter and the output of a 3-D frame comb filter, based on the value of k which is determined by the output signal of a motion detector. For example, if the k value is "0" when no motion is detected, a 3-D Y/C signal separation is performed, while if the k value is "1" when the detected motion is great, a 2-D Y/C signal separation is performed. with respect to the intermediate values between the values "0" and "1", an adaptive Y/C signal separation is performed. In detecting the image motion, the Y component is detected by the one-frame difference value of the low frequency component of the Y signal, while the C component is detected by the difference values between the Y signal of a high frequency band and the C signals of two frames. Alternatively, the C component may be detected by the difference value between the Y signal of the high frequency band and the C signal of one frame.
According to the conventional Y/C signal separating device as described above, however, the horizontal band-pass filter provides a relatively good Y/C signal separation function in the event that a small change of image occurs in a horizontal direction, but has a drawback that the Y/C signal separation is not satisfactorily performed due to the color interference and luminance interference in the event that a large change of image occurs in the horizontal direction, thereby deteriorating a horizontal resolution of the image.
The vertical filter has a drawback that dot crawling and color interference may occur when a correlation in a vertical direction is lowered. Also, the motion-adaptive 3-D comb filter has a drawback that the motion estimation and compensation are not effectively performed, and thus the separation of the Y/C signal is inaccurately performed, thereby causing the luminance interference and color interference to occur.